1. Field of the Invention
The present invention relates to a secondary air supply system for supplying secondary air into the exhaust system of internal combustion engines for the purpose of purifying exhaust gases.
2. Description of the Prior Art
It is apparently known in the prior art to include a secondary air supply system to inject secondary air into the exhaust system of internal combustion engines as a countermeasure for purifying exhaust gases by recombusting harmful components such as HC and CO contained in the exhaust gases by the secondary air. The conventional secondary air supply system generally comprises a rotary air pump such as a vane pump mounted on the internal combustion engine to supply compressed air delivered from the air pump into the exhaust gas passage through an air injection manifold. As an alternative, it has also been proposed to utilize exhaust gas pressure pulsation existing in the exhaust gas passage wherein air is injected into the exhaust system by employing a passage means for conducting air into the exhaust system and a check valve provided in the midst of said passage means. This arrangement permits air to flow only toward the exhaust system so that air is pumped toward the exhaust system each time a vacuum is produced in the exhaust system during pulsation of the exhaust gas pressure. When a suitable capacity air pump is used, the system is capable of feeding any quantity of air required for sufficient purification of exhaust gases over the entire operational region of the engine. However, this system reqiures a relatively large air pump, particularly when it is applied to an engine having a large piston displacement thereby causing a serious problem with regard to the space limit of the engine compartment which has to accommodate other various auxiliary units used in modern automobiles adapted for the new exhaust gas regulations. By contrast, the latter system utilizing the exhaust gas pressure pulsation has an advantage with regard to the space requirement thereof. However, this system has a limitation with regard to its air injection capacity and the capacity is generally insufficient for engines having a relatively large piston displacement. Furthermore, although it is preferable that the quantity of secondary air should be proportional to the quantity of engine intake air so that the supply of secondary air is enhanced as the rotational speed of the engine increases, the supply of air depending upon the exhaust gas pressure pulsation decreases as the rotational speed of the engine increases due to the fact that the amplitude of the exhaust gas pressure pulsation decreases as the rotational speed of the engine increases.